1. Field of the Invention
The present invention relates to a method for charging a battery, and more particularly to a charging method suited for a lead battery carried aboard a vehicle and used for the purpose of starting an engine, turning on a lamp, operating an air conditioner, and the like.
The present invention also relates to a method for determining the life of a battery, and more particularly to a life determination method suited for an on-board lead battery used for the purpose of starting an engine, turning on a lamp, operating an air conditioner, and the like.
The present invention also relates to a method for detecting the state of charge of a lead battery and a method for determining the degradation of a lead battery using the detection method, and more particularly to a state-of-charge detection method and a degradation determination method using the detection method, suited for an on-board lead battery used for the purpose of starting an engine, turning on a lamp, operating an air conditioner, and the like.
2. Description of the Related Art
Batteries such as a lead battery carried aboard a vehicle are used for the purpose of starting an engine, turning on a lamp, and the like. In driving, the batteries are subjected to constant-voltage charge with a predetermined voltage so that the batteries are charged to about 100% or more of full charge. The state of charge of the batteries are hereinafter referred to as SOC. The term xe2x80x9cSOCxe2x80x9d may also refer to a charging rate.
Recently, in order to improve the fuel efficiency of vehicles, a system for charging regenerative power in deceleration into a battery, and such a system further operating an air conditioner or the like, which is conventionally energized by the driving force of an engine, using a lead battery, have been proposed.
In order to perform such regenerative charge, a battery needs to be in a SOC in which the battery is kept partially charged, i.e., less than 100% of full charge.
In a lead battery, when charge and discharge are performed at a low level of SOC, degradation is likely to proceed. The degradation is particularly significant when the SOC is low, specifically when charge and discharge are continuously repeated where the SOC is about 50% or less. In this case, at a positive pole, the sulfuric acid concentration of an electrolyte solution is about 15% or less so that the corrosion rate of a positive pole grid made of lead alloy is rapidly increased while, at a negative pole, coarse lead sulfate is produced as a result of discharge so that the discharge capacity of the lead battery is dramatically reduced.
Therefore, when a lead battery is charged and discharged at a low level of SOC, the level of a SOC needs to be about 50% or more. Further, refresh charge needs to be performed at a certain frequency or more so that the SOC is about 100%.
The SOC of a lead battery is typically evaluated based on an open circuit voltage (OCV) of the lead battery. The SOC and OCV of the lead battery has substantially a constant relationship. When the SOC is decreased, the OCV is linearly decreased. When it is detected that the SOC is a predetermined value or less, the lead battery is subjected to refresh charge. Alternatively, the lead battery is subjected to refresh charge at predetermined time intervals.
The degradation of a lead battery due to a reduction in a SOC is involved with the sulfuric acid concentration of an electrolyte solution. Specifically, when a SOC is about 50% or less, the sulfuric acid concentration of an electrolyte solution is about 16% or less. In such a situation, when charge and discharge are performed, the corrosion rate of a positive pole grid made of lead alloy is rapidly increased. This may be responsible for the degradation of the lead battery.
Therefore, a SOC needs to be measured with precision during charging.
As described above, the SOC and OCV of a lead battery has substantially a constant relationship. When the SOC is decreased, the OCV is linearly decreased. By detecting the SOC in accordance with such a relationship, the lead battery can be controlled so that the SOC falls within an appropriate range.
Lead batteries for use in vehicles have been found to be degraded when regenerative charge and discharge are repeated using a large current density when the SOC is low. Such a degradation in performance is not found in alkaline batteries such as a nickel-hydrogen battery, lithium ion secondary batteries, or the like, but is specific to lead batteries. Due to the performance degradation, the lead batteries are unlikely to provide stable use over a long time.
Further, when a lead battery is used at a low temperature of 0xc2x0 C. or less, the viscosity of the electrolyte solution is rapidly increased. This leads to a reduction in a battery reaction rate, resulting in rapid decreases in a discharge capacity and an output voltage.
Furthermore, when a SOC is about 50% or less, the sulfuric acid concentration of an electrolyte solution is lowered. The corrosion rate of a positive pole grid made of lead alloy is rapidly increased. This leads to a reduction in the life of a lead battery.
When an on-board lead battery is degraded due to use, even if the battery is refresh-charged toward a SOC of 100%, the SOC is not increased up to 100%, i.e., the performance of the lead battery is decreased. If such a degraded lead battery, in which even though the battery is refresh-charged toward a SOC of 100%, the SOC is not increased up to 100%, is further used, the lead battery will soon be dead, i.e., cannot be used. Since by how much the lead battery is degraded cannot be clearly detected, the lead battery may unexpectedly die.
When an on-board lead battery is degraded due to use, the SOC cannot be increased even if the OCV is increased. If the water content of an electrolyte solution is decreased due to overcharge or evaporation, the sulfuric acid concentration of the electrolyte solution is increased. Since the OCV is determined by the sulfuric acid concentration, the OCV is consequently increased. When the degradation further proceeds, coarse lead sulfate is produced as a result of discharge and accumulated at an active material of positive and negative poles. This hinders the SOC from being recovered even when the lead battery is recharged. In this case, the sulfuric acid concentration is decreased, so that the OCV-SOC relationship is deviated from a predetermined condition. In such a case, even though the SOC is evaluated based on the predetermined OCV-SOC relationship, the exact SOC is unlikely to be obtained. This makes it impossible to control charge with high precision so that the lead battery is in a predetermined SOC. Such a problem arises significantly in a lead battery in which sulfuric acid which is a solute in an electrolyte solution acts as a battery active material.
According to one aspect of the present invention, a method for charging a lead battery is provided, in which a SOC is controlled so as to be less than 100%, and when the lead battery has not been charged and discharged for a predetermined time, refresh charge is performed so that the SOC becomes at least 90% or more.
According to another aspect of the present invention, a method for charging a lead battery is provided, in which a SOC is controlled so as to be less than 100%, and every time the lead battery is charged and discharged in predetermined time intervals, refresh charge is performed so that the SOC becomes at least 90% or more.
According to one aspect of the present invention, a method for charging a lead battery is provided, in which a SOC is controlled so as to be less than 100%, and when the SOC is lowered below a predetermined value, refresh charge is performed so that the SOC becomes at least 90% or more.
According to another aspect of the present invention, a method for charging a lead battery is provided, in which a SOC is controlled so as to be less than 100%, and when a temperature of the lead battery is less than or equal to a predetermined value, refresh charge is performed so that the SOC becomes at least 90% or more.
In one embodiment of this invention, in the refresh charge, after a charging current is lowered to a predetermined value even though the lead battery is subjected to constant-voltage charge using a predetermined control voltage, constant-current charge is performed, or constant-voltage charge is performed using a control voltage higher the predetermined control voltage.
In one embodiment of this invention, a set value of the SOC, after the refresh charge, is increased with an increase in a cycle of the refresh charge.
In one embodiment of this invention, the SOC is calculated based on an OCV of the lead battery immediately before the start of charge and discharge of the lead battery.
In one embodiment of this invention, the SOC is calculated based on an OCV of the lead battery when charge and discharge are paused, or based on a discharging current and a discharging voltage.
According to another aspect of the present invention, a method for determining the life of a battery is provided, in which the battery is subjected to refresh charge, a SOC of the battery after the refresh charge is presumed to be 100%, charge and discharge are performed, and a SOC(1) of the battery after the charge and discharge, the SOC(1) being obtained based on a quantity of charged, and a SOC(2) obtained based on a discharging voltage in discharge are compared with each other, and the life of the battery is determined based on a difference between the SOC (1) and the SOC(2).
According to another aspect of the present invention, a method for determining the life of a battery is provided, in which a first DC-IR value is obtained with respect to a SOC of the battery in an initial state where the battery is not degraded, based on charging and discharging currents and a battery voltage, and the first DC-IR value is compared with a second DC-IR value of the battery in the initial state obtained in advance, and the life of the battery is determined based on a difference between the first and second DC-IR values.
According to another aspect of the present invention, a method for detecting a SOC of a lead battery is provided, in which an OCV of the lead battery or a discharging voltage of the lead battery when a discharging current is less than or equal to a predetermined value is measured, and the SOC of the lead battery is detected based on a relationship between a predetermined OCV and the SOC or a relationship between the discharging voltage and the SOC, and when the SOC is calculated, a variation in the OCV or a variation in the discharging voltage and a variation in the SOC are measured with respect to a predetermined time, and the relationship between the predetermined OCV and the SOC or the relationship between the discharging voltage and the SOC is corrected.
In one embodiment of this invention, the variation in the SOC is obtained based on a quantity of charged and discharged electricity obtained by integrating charging and discharging currents in the predetermined time.
According to another aspect of the present invention, a method for determining degradation of a lead battery using the method for detecting a SOC of a lead battery of this invention is provided, in which the degradation of the lead battery is determined based on a corrected relationship between an OCV of the lead battery and the SOC or a corrected relationship between a discharging voltage and the SOC.
In one embodiment of this invention, a ratio of a variation in the OCV or a variation in the discharging voltage to a variation in the SOC is calculated, and the degradation of the lead battery is determined based on the ratio.
In one embodiment of this invention, a value (K) is calculated by dividing the variation in the OCV or the variation in the discharging voltage by the variation in the SOC, and when the value (K) is greater than or equal to a predetermined value, it is determined that the lead battery is degraded.
Thus, the invention described herein makes possible the advantages of providing: (1) a method for charging a lead battery in which a reduction in the performance of the lead battery is suppressed so that the lead battery can be stably used for a long time; (2) a method for determining the life of a battery in which the life of the battery can be exactly determined so that the sudden death of the battery can be avoided; (3) a method for accurately detecting the SOC in order to charge a lead battery with precision so that the SOC becomes a predetermined value; and (4) a method for determining the condition of a degraded lead battery based on the OCV of the lead battery or a relationship between a discharging voltage and the SOC.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.